A test plan is worth creating if there is something that needs documented testing. It may be frustrating to see a test plan with only a few steps, but if you realize the purpose of the necessity of documentation, it doesn't matter how small the test plan is.
You should include whatever it is that needs testing. Whether it's an anticipated UI response to interaction or a back-end component that gets updated, if it needs to be verified that an action occurred or did not occur, it should be included in the test plan.
I work in an agile environment, and sometimes it requires test plans to be updated due to design changes. In an agile environment, that comes with the definition of the strategy. Just be sure to stay on top of changes that need to be made.